I assume the FGW procedure on a failure is to dispatch a fitter in a van to try and fix the train. That seems sensible, but it is going to take some time to reach the train. It seems here that the system after the fitter declares that he cant fix the fault seems to be the issue. For passengers to be stuck on an overcrowded train for 6 hours is simply wrong and should not happen. Passengers shouldnt be left in that position.
Whilst I am sure FGW didnt want to leave their passengers in that situation for hours something went badly wrong. Either the staffs, on site or in the control office, were not empowered to make the suitable decisions or were not allowed, by way of procedure, to make those suitable decisions. I wonder if the chain of command went beyond the on call manager and required higher level authority.
Of course staying on the train is usually best choice for the passengers but there has to come a point where getting the passengers off becomes the best thing to do. I appreciate it is hard to climb down from a train to the track, that not everyone may be able to do that and that I dont know the exact location and geographical features of the site, but getting people off the train seems sensible after so long. Yes, that is a hassle and is going to take time to organise the required blockages, assemble and brief the required staff and then implement the plan but I doubt it would take 6 hours.
I suppose there are two options to get the passengers off; one is walking along the track under escort, a group at a time, to a safe access point where a bus could take them to civilisation. I suppose the other option is to bring another train along side and try and evacuate everyone over some kind of bridge. Both are slow but do at least make it look like you are doing something.
I also wonder how long it took the rescue loco to arrive and remove the rear power car. How soon after this was the train underway? I guess if that was the main delay it might explain why the passengers couldnt be taken off as the line couldnt be blocked to traffic until the rescue loco had been and gone. 6 hours seems an awful long time for one train failure.
Finally, I am assuming the failure location was too remote to allow extra refreshments to be brought to site to keep the passengers fed and watered. It would be very sensible, if possible, to get water and food to the train, especially in hot weather. Yes, that is going to mean man handling it along the track by a chain of blokes from an access point but that has to be better than leaving passengers in difficult conditions.